1. Functions, Procedures, and Abstraction
Dr. José M. Reyes Álamo

2. Status of Assignment and Quizzes
Past Due
Pending
RA1_ComputerConcepts
RA2_PythonCh1Ch2
RA3_PythonCh5Ch7
Quiz1
RA4_PythonCh3Ch6
RA5_PythonCh8Ch10
Labs
Lab1
Lab2
Lab3
Lab4

3. User defined functions Abstraction Reusability
Outline
Functions
Built-in functions
User defined functions
Abstraction
Reusability
Parameters and arguments
Returning values
Variables Scope

4. Functions

5. Functions
From mathematics, a functions perform some operation and returns a result.
Functions hide the details of and operation, to make it easy to use by others
e.g. sqrt() for computing the square root.

6. Function
In the context of programming, a function is a named sequence of statements that performs a computation.
When you define a function, you specify the name and the sequence of statements.
You can invoke the function by name.

7. Built in functions

8. Python provides built-in functions
Type conversion functions

9. A big list of built-in functions

10. Remember the Math module?
Math functions

11. Lots of Python modules available
Built-in modules
The Python Standard Library:
Other modules
PyPI - the Python Package Index
The Python Package Index is a repository of software for the Python programming language.
There are currently packages there.

12. User defined functions

13. Mathematical notation
Consider a function which converts temperatures from Celsius to temperatures in Fahrenheit:
Formula: F = 1.8 *C
Functional notation: F = celsisus2Fahrenheit(C) where
celsius2Fahrenheit(C) = 1.8*C

14. Terminology: parameter “C”
Function definition
Math:
f(C) = 1.8*C
Python:
Terminology: parameter “C”

15. © 2011 Pearson Addison-Wesley. All rights reserved.

16. Definition and calling

17. Triple quoted string in function
A triple quoted string just after the def is called a docstring
docstring is documentation of the function’s purpose, to be used by other tools to tell the user what the function is used for.

18. Abstraction

19. Abstraction
This is and abstract tree. It is not real, but it is something that makes us think about the features of a tree.
The second tree has more details and the third has even more details.
The fourth tree is a pine and has some different features.
Finally we have a forest

20. Abstraction
The ability of dealing with ideas rather than events.
Something that exists only as an idea.

21. Abstraction in computing
A programmer would use abstraction to define general purpose functions.
Abstraction is one of the most important techniques in software engineering as it is used to reduce complexity.

22. Reusability

23. Support divide-and-conquer strategy Abstraction of an operation
Why have functions?
Support divide-and-conquer strategy
Abstraction of an operation
Reuse: once written, use again
Sharing: if tested, others can use
Security: if well tested, then secure for reuse
Simplify code: more readable

24. How to write a function
Does one thing. If it does too many things, it should be broken down into multiple functions (refactored).
Readable. You should be able to read it as well as others.
Reusable. If it performs its task well, you can reuse.

25. More on functions
Complete. A function should check for all the cases where it might be invoked. Check for potential errors.
Not too long. As it does one thing, code is usually succinct.

26. Parameters and arguments

27. Parameters vs argument
A parameter is the variable which is part of the function's definition.
Inside the function, the arguments are assigned to the parameters.
An argument is an expression, value, or literal used when calling the method.

28. © 2011 Pearson Addison-Wesley. All rights reserved.
celsius=25
val=25* = 77.0
© 2011 Pearson Addison-Wesley. All rights reserved.

29. Returning values

30. Return Statement
The return statement indicates the value that is returned by the function.
The return statement is optional. If there is no return statement, the function is often called a procedure.
Procedures are often used to perform duties such as printing output or storing a file

31. Multiple returns in a function
A function can have multiple return statements, but only one will execute.
The first return statement found, executes and ends the function.
Multiple return statements can make your code confusing, therefore should be used carefully.

32. Multiple return statements example

33. Variables scope

34. Local variables
When you create a variable inside a function, it is local, which means that it only exists inside the function. For example:
This function takes two arguments, concatenates them, and prints the result twice. Here is an example that uses it:

35. Local values
When cat_twice terminates, the variable cat is destroyed. If we try to print it, we get an exception:
Parameters are also local. For example, outside cat_twice, there is no such thing as part1 or part2.

36. Summary Function: Built in functions: User defined functions:
Named sequence of statements that performs a computation
Built in functions:
Python provides built-in functions and modules. Lots of extra modules available
User defined functions:
It is also possible to add new functions. A function definition specifies the name of a new function and the sequence of statements that execute when the function is called.
Abstraction:
The ability of dealing with ideas rather than objects or events.

37. Summary Reusability: Parameters and arguments: Returning values:
Functions can be re-used in another program.
Parameters and arguments:
A parameter is the variable which is part of the function’s definition. An argument is the value or expression used when calling the method.
Returning values:
The return statement indicates the value returned by the function. A function with no return statements is often called a procedure.
Variables Scope:
Parameters and variables created inside a function are local, which means that they only exists inside the function

38. Status of Assignment and Quizzes
Past Due
Pending
RA1_ComputerConcepts
RA2_PythonCh1Ch2
RA3_PythonCh5Ch7
Quiz1
RA4_PythonCh3Ch6
RA5_PythonCh8Ch10
Labs
Lab1
Lab2
Lab3
Lab4